Coupling of light from one point to another or one location to another presents several challenges, particularly in optical transmission of information, production of high power coherent light, pump sources for lasers, and the like. The challenges presented are illustrated, for example, in the context of developing high power vertical cavity surface emitting lasers (VCSELs).
High-power VCSEL arrays have been demonstrated by several research groups. Grabherr et al. reported VCSEL power densities exceeding 300 W/cm2 from a 23-element array [M. Grabherr et. al., Electron. Lett., vol. 34, p. 1227, 1998]. Francis et al. demonstrated VCSEL power in excess of 2-W continuous wave and 5 W pulsed from a 1000-element VCSEL array [D. Francis, et. al., IEEE Int. Semiconductor Laser Conf. (ISLC), Nara, Japan, October 1998]. Chen et al. also reported the power density of about 10 kW/cm2 Steradian from an array of 1600 VCSELs using a microlens array to individually collimate light from each laser [H. Chen, et. al., IEEE Photon. Technol. Lett., vol. 11, No. 5, p. 506, May 1999]. However, their beam quality at high power is still poor. A high quality beam requires a narrow linewidth single mode with high spatial and temporal coherence.
In order to produce coherent, single-frequency, high-power arrays of VCSELs, the elements of one or two dimensional VCSEL arrays should be phase-locked. Although the light from each individual VCSEL is coherent, the phase and frequencies (or wavelengths) of the light from each VCSEL are slightly different and therefore uncorrelated. For such an incoherent array comprising N elements producing the same power P, the on-axis power in the far-field is ˜NP. However, if the array as a whole can be made coherent, in phase, and with a single frequency, the on-axis power in the far-field is N2P and the width of the radiation pattern is reduced by ˜1/N. Previous efforts to phase-lock arrays of VCSELs have used diffraction coupling [J. R. Legar, et. al., Appl. Phys. Lett., vol. 52, p. 1771, 1988]. and evanescent coupling. [H. J. Yoo, et. al., Appl. Phys. Lett., vol. 56, p. 1198, 1990]. Diffraction coupling depends on geometrical scattering of light and evanescent coupling requires that the optical field of adjacent array elements overlap. Both approaches impose restrictions on the array architecture. More importantly, these existing approaches have had very limited success, even in 1D edge-emitting arrays where both approaches have been extensively investigated. Recently, Choquette et al. has demonstrated phase locking in a VCSEL array using an anti-guide approach [D. K. Serkland, et. al., IEEE LEOS Summer Topical Meeting, p. 267, 1999].
A further challenge involves coupling light out of a source, such as a VCSEL. Such coupling may, for example, adversely affect the amount of light which is available to be coupled out of the VCSEL. Accordingly, it would be desirable to provide an efficient waveguide coupling structure, particularly for use with a VCSEL.